Photoinduced electron transfer in non-aqueous microemulsions

Photoinduced electron transfer from zinc tetraphenyl porphyrin (ZnTPP) inco rporated in n-heptane/AOT/ethylene glycol microemulsions was followed by la ser flash photolysis and fluorescence quenching. Using two acceptors, duroq uinone (DQ) and methyl viologen (MV2+) which are located on opposite sides of the interfacial region, the apolar and polar pseudophases respectively, it was possible to monitor kinetic and spectroscopically the respective rad ical ions formed. The determination of local quencher concentrations enable d the evaluation of electron transfer quenching rate constants in each pseu dophase. The values obtained showed that when both the fluorophore and the quencher are either in the oil pseudophase or at the interface the processe s are diffusion-controlled limited. The magnitude of the rate constants ran ges from 10(8) to 10(10) mol(-1) dm(3) s(-1). By contrast, the forward elec tron transfer occurring in the polar pool is reaction controlled (k(q)(T) = 2.1 x 10(6) mol(-1) dm(3) s(-1)) whereas the back recombination of radical ions in the pool is also diffusion controlled (k(2) = 4.1 x 10(8) mol(-1) dm(3) s(-1)). The triplet state kinetics is well supported by steady-state and transient fluorescence quenching studies from which effective reactional distances (9 -12 Angstrom) and diffusion coefficients (0.5-1.3) x 10(-9) m(-2) s(-1), co uld be evaluated at both the oil and interface pseudophases. The larger eff ective reaction distances coupled with lower diffusion coefficients estimat ed at the interfacial region connected to the polar non-aqueous solvent sho ws that factors such as the distance, mutual orientation and microviscosity are the controlling physical parameters. On the other hand, beyond the ene rgetics, the efficiency of the whole electron transfer in the inner polar n on-aqueous nanophase depends on the solvation of radical ions formed. (C) 2 001 Elsevier Science B.V. All rights reserved.